What does maximum resident set size mean?

Question

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
  int i = 0;
  struct rusage r_usage;
  while (++i <= 10) {
    void *m = malloc(20*1024*1024);
    memset(m,0,20*1024*1024);
    getrusage(RUSAGE_SELF,&r_usage);
    printf("Memory usage = %ld\n",r_usage.ru_maxrss);
    sleep (3);
  }
  printf("\nAllocated memory, sleeping ten seconds after which we will check again...\n\n");
  sleep (10);
  getrusage(RUSAGE_SELF,&r_usage);
  printf("Memory usage = %ld\n",r_usage.ru_maxrss);


  return 0;
}

The above code uses ru_maxrss attribute of rusage structure. It gives the value of maximum resident set size. What does it mean? Each time the program is executed it gives a different value. So please explain the output of this code?

These are screenshots of two executions of the same code which give different outputs, how can these numbers be explained or what can be interpreted from these two outputs?

Answer 1

Resident set size (RSS) means, roughly, the total amount of physical memory assigned to a process at a given point in time. It does not count pages that have been swapped out, or that are mapped from a file but not currently loaded into physical memory.

"Maximum RSS" means the maximum of the RSS since the process's birth, i.e. the largest it has ever been. So this number tells you the largest amount of physical memory your process has ever been using at any one instant.

It can vary from one run to the next if, for instance, the OS decided to swap out different amounts of your program's memory at different times. This decision would depend in part on what the rest of the system is doing, and where else physical memory is needed.